The size of wireless communication devices is being driven towards smaller sizes while the desire to incorporate additional features into such devices continues to increase. Communication devices, such as portable two-way radios, which operate over different frequency bands are considered desirable, particularly in the public-safety arena where such devices are used by different agencies such as police departments, fire departments, emergency medical responders, and military, to name a few, which may own systems operating in different frequency bands. Thus the need for reliable inter-agency communications in emergency situations drives the need for wireless communication devices that enable reliable interoperability across systems. The use of separate antennas to cover different frequency bands is often not a practical option in view of the portability and size limitations of such devices, as well as the mentioned interoperability requirement.
One particularly useful combination of bands desirable to achieve in an portable two-way radio antenna comprises a very high frequency (VHF) band (about 136-174 MHz), an ultra high frequency (UHF) band (about 380-520 MHz), and a 7/800 MHz band (about 764-869 MHz). Other bands could also be desirable, for instance a global positioning system (GPS) band (about 1565-1585 MHz) or a long-term evolution (LTE) public-safety band (about 758-798 MHz). Furthermore, due to the need of emergency personnel to carry a portable two-way radio during an entire work shift and to operate effectively in dangerous environments, problems with antenna stiffness and overall size must be considered in such a design.
It is especially challenging to combine the above referenced bandwidths into a single structure. To be an effective radiator, antennas (also called radiating elements) normally have electrical lengths equal to, or some multiple of, a quarter of the wavelength λ. A good compromise between length and radiating performance for many portable radios is λ/4. Thus, a VHF radiating element designed according to this criterion has a relatively long physical length of about 50 cm at the center of the VHF band, while the UHF radiating element of λ/4 is about 18 cm, and the 7/800 MHz radiating element electrical length of λ/4 is about 9 cm. Creating a single length antenna that works efficiently at these disparate frequencies, while also minimizing the overall length and maximizing its flexibility, is difficult.
Accordingly, it is desirable to provide a multi-band antenna structure while retaining a relatively small form factor.
Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of embodiments of the present invention.
The apparatus and method components have been represented where appropriate by conventional symbols in the drawings, showing only those specific details that are pertinent to understanding the embodiments of the present invention so as not to obscure the disclosure with details that will be readily apparent to those of ordinary skill in the art having the benefit of the description herein.